Phase change memory cell and manufacturing method thereof using minitrenches

ABSTRACT

A process forms a phase change memory cell using a resistive element and a memory region of a phase change material. The resistive element has a first thin portion having a first sublithographic dimension in a first direction; and the memory region has a second thin portion having a second sublithographic dimension in a second direction transverse to the first dimension. The first thin portion and the second thin portion are in direct electrical contact and define a contact area of sublithographic extension. The second thin portion is delimited laterally by oxide spacer portions surrounded by a mold layer which defines a lithographic opening. The spacer portions are formed after forming the lithographic opening, by a spacer formation technique.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a phase change memory cell and to amanufacturing process thereof.

2. Description of the Related Art

As is known, phase change memory (PCM) elements exploit thecharacteristics of materials which have the property of changing betweentwo phases having distinct electrical characteristics. For example,these materials may change from an amorphous phase, which is disorderly,to a crystalline or polycrystalline phase, which is orderly, and the twophases are associated to considerably different resistivity.

At present, alloys of group VI of the periodic table, such as Te or Se,referred to as chalcogenides or chalcogenic materials, canadvantageously be used in phase change cells. The chalcogenide thatcurrently offers the most promise is formed by a Ge, Sb and Te alloy(Ge₂Sb₂Te₅), which is currently widely used for storing information inoverwritable disks.

In chalcogenides, the resistivity varies by two or more magnitude orderswhen the material passes from the amorphous phase (more resistive) tothe polycrystalline phase (more conductive) and vice versa. Thecharacteristics of chalcogenides in the two phases are shown in FIG. 1.As may be noted, at a given read voltage, here designated by Vr, thereis a resistance variation of more than 10.

Phase change may be obtained by locally increasing the temperature, asshown in FIG. 2. Below 150° C. both phases are stable. Above 200° C.(temperature of start of nucleation, designated by T_(x)), fastnucleation of the crystallites takes place, and, if the material is keptat the crystallization temperature for a sufficient length of time (timet₂), it changes its phase and becomes crystalline. To bring thechalcogenide back into the amorphous state, it is necessary to raise thetemperature above the melting temperature T_(m) (approximately 600° C.)and then to cool the chalcogenide off rapidly (time t₁).

From the electrical standpoint, it is possible to reach both criticaltemperatures, namely the crystallization temperature and the meltingpoint, by causing a current to flow through a resistive element whichheats the chalcogenic material by the Joule effect.

The basic structure of a PCM element 1 which operates according to theprinciples described above is shown in FIG. 3 and comprises a resistiveelement 2 (heater) and a programmable element 3. The programmableelement 3 is made of a chalcogenide and is normally in thepolycrystalline state in order to enable a good flow of current. Onepart of the programmable element 3 is in direct contact with theresistive element 2 and forms the area affected by phase change,hereinafter referred to as the phase change portion 4.

If an electric current having an appropriate value is caused to passthrough the resistive element 2, it is possible to heat the phase changeportion 4 selectively up to the crystallization temperature or to themelting temperature and to cause phase change. In particular, if acurrent I flows through a resistive element 2 having resistance R, theheat generated is equal to I²R.

The use of the PCM element of FIG. 3 for forming memory cells hasalready been proposed. In order to prevent noise caused by adjacentmemory cells, the PCM element is generally associated to a selectionelement, such a MOS transistor, a bipolar transistor, or a diode.

All the known approaches are, however, disadvantageous due to thedifficulty in finding solutions that meet present requirements asregards capacity for withstanding the operating currents and voltages,as well as functionality and compatibility with present CMOStechnologies.

In particular, considerations of a technological and electrical natureimpose the creation of a contact area of small dimensions, preferably 20nm×20 nm, between the chalcogenic region and a resistive element.However, these dimensions are much smaller than those that can beobtained with current optical (UV) lithographic techniques, whichscarcely reach 100 linear nm.

BRIEF SUMMARY OF THE INVENTION

An embodiment of the present invention provides a phase change memorycell that includes a resistive element, including a first thin portionhaving a first sublithographic dimension in a first direction, and amemory region of a phase change material and including a second thinportion having a second sublithographic dimension in a second directiontransverse to the first direction. The resistive element and the memoryregion are in direct electrical contact at the first and second thinportions and define a contact area of sublithographic extension. Thesecond thin portion is delimited laterally in the second direction byspacer portions of a first dielectric material, thereby defininginclined surfaces in a third direction, transverse to said first andsecond directions.

Another embodiment of the invention provides a process for manufacturinga phase change memory cell. The process includes forming a resistiveelement including a first thin portion having a first sublithographicdimension in a first direction; and forming a memory region of a phasechange material and including a second thin portion having a secondsublithographic dimension in a second direction transverse to the firstdirection. The first and second thin portions define a contact area ofsublithographic extension. Forming a memory region includes forming amold layer on top of the resistive element, forming a first lithographicopening in the mold layer, forming spacer portions in the firstlithographic opening, the spacer portions defining a slit having thesecond sublithographic dimension; and depositing a phase change layerinside the slit.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, a preferredembodiment thereof is now described, purely by way of non-limitingexample, with reference to the attached drawings, in which:

FIG. 1 shows the current versus voltage characteristic of a phase changematerial;

FIG. 2 shows the temperature versus current plot of a phase changematerial;

FIG. 3 shows the basic structure of a PCM memory element;

FIG. 4 shows a cross section of a wafer of semiconductor material in amanufacturing step of the cell of FIG. 3, according to the parent patentapplication;

FIG. 5 shows the layout of some masks used for forming the structure ofFIG. 4;

FIG. 6 is a cross-section taken along line VI-VI of FIG. 5;

FIGS. 7-14 are cross-section of the structure of the above mentionedpatent application, in successive manufacture steps;

FIG. 15 is a top plan view, with parts removed and at an enlarged scale,of a detail of FIG. 4;

FIGS. 16 a and 16 b are top plan views, with parts removed, of a detailof FIG. 14, in two different manufacture conditions;

FIG. 17 shows the layout of some masks used for forming the structure ofFIG. 7, according to an embodiment of the invention;

FIG. 18 is a cross-section similar to FIG. 8, in a manufacture stepaccording to an embodiment of the invention;

FIG. 19 shows the layout of some masks used for forming the structure ofFIG. 18;

FIGS. 20 and 21 are cross-sections, similar to FIG. 18, in successivemanufacture steps according to an embodiment of the invention;

FIG. 22 is a top plan view of the structure of FIG. 21;

FIG. 23 is a cross-section, similar to FIG. 21, in a subsequentmanufacture step;

FIG. 24 shows the layout of same masks used for forming the structure ofFIG. 23;

FIG. 25 is a cross-section, similar to FIG. 14, in a final manufacturestep according to an embodiment of the invention;

FIGS. 26 a and 26 b are top plan views of the contact area, in twodifferent manufacture conditions; and

FIGS. 27 and 28 show two steps regarding a different embodiment.

DETAILED DESCRIPTION OF THE INVENTION

The parent application teaches forming the contact area as anintersection of two thin portions extending transversely with respect toone another and each of a sublithographic size. In order to form thethin portions, deposition of layers is adopted instead of a lithographicprocess, given that deposition makes it possible to obtain very thinlayers, i.e., having a thickness much smaller than the current minimumsize that can be achieved using lithographic techniques.

For a better understanding of embodiments of the present invention, themanufacturing process of the parent patent application will now bedescribed.

With reference to FIG. 4, initially a wafer 10 comprising a P-typesubstrate 11 is subjected to standard front end steps. In particular,inside the substrate 11 insulation regions 12 are formed and delimitactive areas 16; then, in succession, N-type base regions 13, N⁺-typebase contact regions 14, and P⁺-type emitter regions 15 are implanted.The base regions 13, base contact regions 14, and emitter regions 15form diodes that form selection elements for the memory cells.

Next, a first dielectric layer 18 is deposited and planarized; openingsare formed in the first dielectric layer 18 above the base contactregions 13 and emitter regions 15, and the openings are filled withtungsten to form base contacts 19 b and emitter contacts 19 a. The basecontacts 19 b are thus in direct electrical contact with the basecontact regions 13, and the emitter contacts 19 a are in directelectrical contact with the emitter regions 15. Advantageously, theopenings in the first dielectric layer 18 can be covered by a barrierlayer, for example a Ti/TiN layer, before being filled with tungsten. Inthis way, the structure of FIG. 4 is obtained.

FIG. 5 shows the layout of some masks used for forming the structure ofFIG. 4 regarding a pair of memory cells 5 that are adjacent in aperpendicular direction to the sectional plane of FIG. 4 (Y direction).In particular, the figure shows a mask A used for defining the activeareas 16, a mask B used for implanting the emitter regions 15, and amask C for forming the openings where the base contacts 19 b and theemitter contacts 19 a are to be formed. FIG. 4 is a cross-section takenalong line IV-IV of FIG. 5, while FIG. 6 shows the same structuresectioned along the section line VI-VI of FIG. 5.

Next (FIG. 7), a second dielectric layer 20—for example, an undopedsilicon glass (USG) layer—is deposited, and openings 21 are formed inthe second dielectric layer 20 above the emitter contact 19 a. Theopenings 21 have dimensions dictated by the lithographic process andare, for example, circle-shaped. Next, a heating layer, for example ofTiSiN, TiAIN or TiSiC, is deposited for a thickness of 10-50 nm,preferably 20 nm. The heating layer, designed to form the resistiveelement 2 of FIG. 3, conformally coats the walls and bottom of theopenings 21 and is subsequently removed outside the openings 21. Theremaining portions of the heating layer thus form a cup-shaped region 22and are then filled with dielectric material 23.

Next, as shown in the enlarged detail of FIG. 8, a mold layer 27, forinstance USG having a thickness of 20 nm, an adhesion layer 28, forinstance Ti or Si with a thickness of 5 nm, and a first delimiting layer29, for example nitride or another material that enables selectiveetching with respect to the adhesion layer 28, are deposited insequence. The first delimiting layer 29 has a thickness of, forinstance, 150 nm. Then, using a mask, one part of the first delimitinglayer 29 is removed by dry etching to form a step which has a verticalside 30 that extends vertically on top of the dielectric material 23.The structure shown in FIG. 8 is thus obtained.

Next (FIG. 9), a sacrificial layer 31, for example TiN with a thicknessof 30 nm, is deposited conformally. In particular, the sacrificial layerforms a vertical wall 31 a that extends along the vertical side 30 ofthe first delimiting layer 29.

Next (FIG. 10), the sacrificial layer 31 is undergoes an etch back thatresults in removal of the horizontal portions of the sacrificial layer31 and of part of the vertical wall 31 a. By appropriately choosing thethickness of the first delimiting layer 29 and the thickness of thesacrificial layer 31, as well as the time and type of etching, it ispossible to obtain the desired sublithographic width W1 for the bottompart of the remaining vertical wall 31 a.

As shown in FIG. 11, a second delimiting layer 35, of the same materialas the first delimiting layer 29, for example nitride, with a thicknessof 300 nm, is deposited. Next, the delimiting layers 29, 35 and thevertical wall 31 a are thinned by chemical mechanical polishing (CMP).At the end, the remaining portions of the delimiting layers 29, 35 forma hard mask, and the remaining portion of the vertical wall forms asacrificial region 36.

Next (FIG. 12), the sacrificial region 36 is removed. The adhesion layer28 is isotropically etched, and the mold layer 27 is dry etched to forma slit 37 in the mold layer 27, the slit 37 having a width W1 equal tothe width of the sacrificial region 36.

Next (FIG. 13), the delimiting layers 29, 35 are removed, and achalcogenic layer 38, for example of Ge₂Sb₂Te₅ with a thickness of 60nm, is deposited conformally. The portion 38 a of the chalcogenic layer38 fills the slit 37 and forms, at the intersection with the cup-shapedregion 22, a phase change region similar to the phase change portion 4of FIG. 3. Then, on top of the chalcogenic layer 38 a barrier layer 39,for example of Ti/TiN, and a metal layer 40, for example of AlCu, aredeposited. The structure of FIG. 13 is thus obtained.

Next (FIG. 14), the stack formed by the metal layer 40, barrier layer39, chalcogenic layer 38, and adhesion layer 28 is defined using a samemask to form a bit line 41. Finally, a third dielectric layer 42 isdeposited, which is opened above the base contacts 19 b. The openingsthus formed are filled with tungsten to form top contacts 43 in order toprolong upwards the base contacts 19 b. Then standard steps areperformed for forming the connection lines for connection to the basecontacts 19 b and to the bits lines 41, and the final structure of FIG.14 is thus obtained.

In practice, as shown in FIG. 15, the intersection between thecup-shaped region 22 and the thin portion 38 a of the chalcogenic layer38 forms a contact area 45 which is approximately square and hassublithographic dimensions. This is due to the fact that both thecup-shaped region 22 and the thin portion 38 a have a width equal to thethickness of a deposited layer. In fact, the width of the cup-shapedregion 22 is given by the thickness of the heating layer, and the widthof the thin portions 38 a is determined by the thickness of thesacrificial layer 31 along the vertical side 30. In greater detail, inthe proximity of the contact area 45, the cup-shaped region 22 has asublithographic dimension in a first direction (Y direction), and thethin portion 38 a has a sublithographic dimension (width W1 of FIG. 10)in a second direction (X direction) which is transverse to the firstdirection. Hereinafter, the term “sublithographic dimension” means alinear dimension smaller than the limit dimension achievable withcurrent optical (UV) lithographic techniques, and hence smaller than 100nm, preferably 50-60 nm, down to approximately 20 nm.

In the process described above, forming the thin portion 38 a of thechalcogenic layer 38 entails numerous steps and is somewhat complex.Consequently, it is desirable to avail a simpler alternative process.

In addition, the dimensions of the contact area 45 depend upon thealignment tolerances between the mask used for forming the openings 21and the mask used for removing part of the first delimiting layer 29 andfor forming the vertical side 30 (FIG. 8). In fact, as emerges clearlyfrom a comparison between FIGS. 16 a and 16 b which are top plan viewsof the contact area 45, in the case of a cup-like region 22 having acircular shape and a diameter of approximately 0.2 an alignment error ofeven only 0.05 μm between the two masks results in the thin portions 38a no longer crossing the cup-shaped regions 22 perpendicularly, with aconsequent considerable increase in the dimensions of the contact area45 (see FIG. 16 b) and hence a considerable increase in the flowingcurrent, the value whereof would be uncontrollable.

Furthermore, the thin portion 38 a crosses each cup-shaped region 22 intwo points, thus doubling the total contact area between the thinportions 38 a and the cup-shaped regions 22, and consequently alsoincreasing the programming current. In the case of a marked misalignmentbetween the two above masks, just one contact area is even obtainedwhich has dimensions far greater than the requirements. The presence ofa double contact gives rise to functional problems, given that in thissituation it would be impossible to know which of the two contact areas45 first causes switching of the overlying thin portion 38 a (i.e., thephase change portion), nor would it be possible to be certain that bothof the thin portions 38 a overlying the two contact areas will switch.

In the following description, parts that are the same as thosepreviously described with reference to FIGS. 4-14 are designated by thesame reference numbers.

The process according to an embodiment of the present inventioncomprises initial steps equal to those described above, up to depositionof the second dielectric layer 20 (FIG. 7). Next, also here the openings21 and the cup-shaped regions 22 are formed. However, as shown in FIG.17, for the definition of the openings 21, a heater mask D is used whichhas rectangular windows (the term “rectangular” also comprising theparticular case of a square shape). Consequently, the openings 21 have asubstantially rectangular shape. Then the heating layer, for example ofTiSiN, TiAIN or TiSiC, with a thickness of 10-50 nm, preferably 20 nm,is deposited. The heating layer coats the walls and bottom of theopenings 21 conformally. Consequently, in top plan view, the cup-likeregions 22 here define an ideally rectangular shape, possibly withrounded edges (on account of the lithographic limits), or at the most anovalized shape, with the longer side, or main direction, parallel to theX direction (FIG. 22). Next, the heating layer is removed outside theopenings 21 to form the cup-shaped regions 22, which are then filledwith the dielectric material 23.

Then (FIG. 18), a stop layer 48, for example of nitride deposited byPECVD (Plasma Enhanced Chemical Vapor Deposition) with a thickness of 40nm, a mold layer 49, for example of USG deposited by PECVD or SACVD(Sub-Atmospheric Chemical Vapor Deposition) with a thickness of 50-70nm, and an adhesion layer 50, for example of Ti or Si with a thicknessof 20-40 nm, are deposited in sequence.

Next, using a minitrench mask, designated by E in FIG. 19, the adhesionlayer 50, the mold layer 49 and the stop layer 48 are etched. As shownin FIG. 19, the minitrench mask E has a rectangular window that extendsbetween two adjacent cells 5 in the Y direction (perpendicular to thealignment direction of the base and emitter regions 14, 15 of eachmemory cell 5, FIG. 7).

Following upon etching, part of the layers 48, 49 and 50 is removed, soas to form an opening 51 having a rectangular shape, corresponding tothat of the minitrench mask E. The width of the opening 51 in the Xdirection is, for example, 160 nm. The opening 51 uncovers part of thedielectric material 23 of the two adjacent cells 5 and crosses eachcup-shaped region 22 only once, as can be clearly seen from thesuperposition of the heater mask D and minitrench mask E in FIG. 19.

Next, FIG. 20, a spacer layer 55, for example an oxide layer, isdeposited (in particular, TEOS with a thickness of 50 nm) is deposited.The spacer layer 55 covers the adhesion layer 50, as well as the wallsand bottom of the opening 51.

Then, FIG. 21, the spacer layer 55 is anisotropically etched by etchingback until the horizontal portions thereof are removed, according to thewell known spacer formation technique. The spacer layer 55 is thencompletely removed above the adhesion layer 50 and is partially removedfrom the bottom of the opening 51 to form a spacer region 55 a whichextends along the vertical sides of the opening 51 (along the perimeterof a rectangle or of an oval) and delimits a slit 56, the base whereofforms a rectangular strip 57 having a sublithographic width W2 (in the Xdirection) of approximately 60 nm. FIG. 22 is a top plan view of thestructure thus obtained, and highlights how the strip 57 uncovers onlyone portion of the cup-shaped region 22 of each cell 5, shown withdashed line in the figure. The uncovered portion of each cup-shapedregion 22 forms a contact area 58, as will be explained hereinafter.

Next, FIG. 23, the chalcogenic layer 38 (also in the present case, forinstance, of Ge₂Sb₂Te₅ with a thickness of 60 nm), the barrier layer 39,and the metal layer 40 are deposited in succession, to form a stack oflayers 41. The chalcogenic layer 38 is in direct contact with theadhesion layer 50, to which it adheres properly, and fills the slit 56with a thin portion 38 a. In particular, the thin portion 38 a of thechalcogenic layer 38 deposits on the strip 57, contacting the cup-shapedregions 22 at the contact areas 58. The inclined wall formed by thespacer region 55 a favors filling of the slit 56, so preventing problemslinked to a poor aspect ratio of the opening 51.

Next, the stack of layers 41 is defined using a stack mask F (FIG. 24).

The process continues with the steps described previously, whichcomprise deposition of the third dielectric layer 42, opening of thethird dielectric layer 42 above the base contacts 19 b, formation of thetop contacts 43, and formation of connection lines for connection to thebase contacts 19 b and to the bit lines 41, so as to obtain the finalstructure shown in FIG. 25.

The advantages of the process and structure described herein areillustrated hereinafter. First, the sequence of steps required forforming the thin portion 38 a is simplified, and the chalcogenic layer38 adheres perfectly to the underlying layers and fills the opening 51correctly, thanks to the inclination of the spacer region 55 a, asalready mentioned previously.

Furthermore, the shape of the minitrench mask E makes it possible toobtain a single contact area 58 for each cup-shaped region 22, and hencefor each cell 5, without requiring any additional masking steps.

The rectangular or ovalized shape of the cup-shaped region 22 reducesthe spread in the dimensions of the contact area 58 also when its shape,instead of being rectangular as in the ideal case, is oval, as may beseen from a comparison between FIG. 26 a, which shows the position ofthe cup-shaped region 22 with respect to thin region 38 a in the absenceof mask misalignment, and FIG. 26 b, which shows the relative positionin presence of misalignment.

Finally, it is clear that numerous modifications and variations may bemade to the process and to the memory cell described and illustratedherein, all falling within the scope of the invention, as defined in theattached claims. For example, the sequence of steps required for formingthe spacer region 55 a and of the strip 57 may vary. In particular, forforming the opening 51 it is possible to etch the adhesion layer 50 andthe mold layer 49 alone, without removing the stop layer 48. Next, thespacer region 55 a is formed in the way described previously, bydepositing a spacer layer and etching it anisotropically. Finally, thestop layer 48 is removed only where it is not covered by the spacerregion 55 a, and in this way the strip 57 is uncovered.

In addition, according to a different embodiment, after forming theopening 51 (FIG. 18) and before depositing the spacer layer 55, aprotective layer 54, of silicon nitride, may be deposited, as shown inFIG. 27. The protective layer 54, preferably deposited by PECVD, has,for instance, a thickness of between 20 and 30 nm. Next, etching back isperformed to remove the horizontal portions of the spacer layer 55 andthen the horizontal portions of the protective layer 54. A protectiveportion 54 a thus is left only beneath the spacer region 55 a, as shownin FIG. 28. The protective layer 54 protects the adhesion layer 50 andprevents contamination thereof by the spacer layer 55, which is ofoxide, both at the top and at the sides. In addition, it functions as anetch stop and prevents undesired etching of the dielectric layer 20 andof the dielectric material 23 during etching back for forming the spacerregion 55 a.

According to a further embodiment, after depositing the adhesion layer50 and before etching using the minitrench mask E, a further nitridelayer having a thickness of 20-30 nm is deposited. Then, using theminitrench mask E, the further nitride layer, the adhesion layer 50, andthe mold layer 49 are selectively removed, without the stop layer 48being removed. The spacer layer 55 is deposited, and an etch back isperformed for forming the spacer region 55 a. Next, a nitride etch iscarried out, removing the horizontal portions of the further nitridelayer above the adhesion layer 55, and the exposed portion of the stoplayer 48. Then the other steps of depositing the chalcogenic layer 38,and so forth, follow. In this way, the further nitride layer protectsthe adhesion layer 50 from any possible contamination by the spacerlayer 55.

All of the above U.S. patents, U.S. patent application publications,U.S. patent applications, foreign patents, foreign patent applicationsand non-patent publications referred to in this specification and/orlisted in the Application Data Sheet, are incorporated herein byreference, in their entirety.

1. A process for manufacturing a phase change memory cell, comprising: forming a resistive element including a first thin portion having a first sublithographic dimension in a first direction; and forming a memory region of a phase change material and including a second thin portion having a second sublithographic dimension in a second direction transverse to said first direction; said first and second thin portions defining a contact area of sublithographic extension; wherein said step of forming the memory region includes forming a mold layer on top of said resistive element; forming a first lithographic opening in said mold layer; forming spacer portions in said first lithographic opening, said spacer portions defining a slit having said second sublithographic dimension; and depositing a phase change layer inside said slit.
 2. The process according to claim 1 wherein said spacer portions are of a first dielectric material and have surfaces inclined in a third direction transverse to said first and second directions, and said mold layer is of a second dielectric material.
 3. The process according to claim 2 wherein, before said step of forming a mold layer, a stop layer of a third dielectric material is formed on top of said resistive element.
 4. The process according to claim 3 wherein said spacer portions are of silicon dioxide, said mold layer is of a silicon glass, and said stop layer is of silicon nitride.
 5. The process according to claim 1 wherein said step of forming a resistive element comprises forming a second lithographic opening in an insulating layer, depositing a conductive layer on a side wall of said second lithographic opening, and filling said second lithographic opening.
 6. The process according to claim 1 wherein said second thin portion has a substantially elongated shape and extends parallel to said first direction.
 7. The process according to claim 1 wherein said resistive element has a cup-like shape and has a vertical side that extends, in top plan view, according to a closed line chosen between a rectangular line and an elongated oval line.
 8. The process according to claim 1 wherein said step of forming spacer portions comprises, after said step of forming a first lithographic opening, the steps of depositing a spacer layer and anisotropically etching said spacer layer.
 9. The process according to claim 1 wherein, before said step of forming a first lithographic opening, the step of depositing an adhesion layer is carried out.
 10. The process according to claim 9 wherein said step of forming spacer portions comprises depositing a protective layer, depositing a spacer layer, anisotropically etching said spacer layer, and selectively removing said protection layer above said adhesion layer and at sides of said spacer portions in said first lithographic opening. 11-20. (canceled)
 21. A process of forming integrated phase change memory cell, comprising: forming a heating element including a first thin portion having a first dimension in a first direction; forming a mold layer on the heating element, the mold layer having a first opening; forming spacer portions in the first opening, the spacer portions defining a slit having a second dimension in a second direction transverse to the first direction; and forming a phase change layer having a second thin portion positioned inside of the slit, the first and second thin portions being in contact with each other at a contact area.
 22. The process of claim 21 wherein the spacer portions are of a first dielectric material and have surfaces inclined in a third direction transverse to the first and second directions, and the mold layer is of a second dielectric material.
 23. The process of claim 22, further comprising forming a stop layer of a third dielectric material between the heater element and the mold layer.
 24. The process of claim 23 wherein the spacer portions are of silicon dioxide, the mold layer is of a silicon glass, and the stop layer is of silicon nitride.
 25. The process of claim 21, wherein forming the heating element includes: forming an insulating layer having a second opening; forming a conductive layer on a side wall of the second opening, the conductive layer being the heating element; and filling the second opening with a material that is different than the conductive layer.
 26. The process of claim 21 wherein the second thin portion has a substantially elongated shape and extends parallel to the first direction.
 27. The process of claim 21 wherein the heating element has a cup-like shape and has a vertical side that extends, in top plan view, according to a closed line shaped between a rectangular line and an elongated oval line.
 28. The process of claim 21, further comprising forming an adhesion layer between the mold layer and the phase change layer. 29-32. (canceled) 